1. Field of the Invention
This invention relates to improvements in a track guide bearing having a number of balls rollably disposed between a ball rolling groove provided axially of an elongate track shaft and an axial ball rolling groove provided in a slide table crowned by the track shaft.
2. Description of the Prior Art
In such a track guide bearing, the slide table is guided through the rolling movement of balls and therefore slide resistance is very small, and the stick scrip phenomenon which is liable to occur to the conventional sliding guide hardly occurs. Also, the balls roll while fitting between the concave grooves of the ball rolling groove and, therefore, such track guide bearing has an advantage that the load capacity is great as compared with that of conventional linear ball bearing. Thus, the track guide bearing has recently become used as the guide element of a machine tool, a measuring instrument or the like, particularly as the guide element of a machine which effects positioning by a pulse motor.
Generally, a carriage apparatus used in a machine tool is great in weight and also the extraneous forces which act thereon may be rather large. Therefore the carriage apparatus of the machine tool requires a track guide bearing of great load capacity and great rigidity, but the conventional track guide bearing has suffered from the disadvantage that it is low in the capability of supporting a high load.
Typical track guide bearings of the prior art will hereinafter be described by reference to FIGS. 6 and 7 of the accompanying drawings.
This track guide bearing is bilaterally symmetric with respect to a vertical axis of symmetry and therefore, FIG. 6 is a fragmentary longitudinal cross section showing only the right upper portion of the track guide bearing.
In the right upper portion of a track shaft 1, arcuate ball rolling grooves 2 and 3 are axially formed in the upper surface and the side surface. In the inner surface of the right upper portion of a slide table 4, ball rolling grooves 5 and 6 corresponding to the ball rolling grooves 2 and 3, respectively, of the track shaft are formed on the opposite sides of a U-shaped groove 7. A number of balls 8 and 9 are rollably fitted between the corresponding grooves of the ball rolling grooves 5, 6 of the slide table and the ball rolling grooves 2, 3 of the track shaft.
In this track guide bearing, the balls 8 and 9 are in contact with the ball rolling grooves 2, 5 or 3, 6 at two points, and the lines of contact intersect each other at a point P. In such a construction, most of the forces acting vertically downwards from the slide table are supported by the upper balls 8 of the track shaft, and the lower balls 9 hardly support these forces. As regards the forces in the upwards direction tending to raise the slide table, these forces are supported by the lower balls 9, and the upper balls 8 are in idle condition. As regards the lateral forces which act on the slide table, the lower balls 9 support almost all of such forces and the upper balls 8 hardly support such forces. Accordingly, the track guide bearing of the prior art having two rows of balls disposed at the left and right corners has utilized only the supporting capability of one row of balls and thus, has been low in load supporting capability.
In another form of the conventional track guide bearing shown in FIG. 7, the load supporting capability is low as in the track guide bearing of FIG. 6. That is, ball rolling grooves 12 and 13 of arcuate cross section are formed at the opposite corners of a ridge provided in the side surface of a track shaft 11. Ball rolling grooves 15 and 16 corresponding to the ball rolling grooves 12 and 13, respectively, of the track shaft are formed in the inner side surface of a slide table 14, and a number of balls 18 and 19 are rollably fitted between these corresponding ball rolling grooves 12, 15 and 13, 16. In this track guide bearing, the lines of contact of the balls 18, 19 intersect each other at a point Q, and the lower balls 19 are idle relative to the vertically downward force which act on the slide table 14 and the upper balls 18 are idle relative to the forces in the direction of raising the slide table and therefore, the load supporting capability is low.
In this track guide bearing of FIG. 7, the load supporting capabilities in the direction of raising and the lateral direction are improved over those in the track guide bearing of FIG. 6, but the track guide bearing of FIG. 7 has a supporting capability of only about 70% for the vertically downwardly acting force.